Add DEVICE_RTC symbol check in RTC source files for NCS36510 target.

targets/TARGET_ONSEMI/TARGET_NCS36510/exceptions.c

@@ -117,11 +117,13 @@ void fIrqUart2Handler(void)
     Uart2_Irq();
 }
 
+#ifdef DEVICE_RTC
 /** Call the RTC IRQ handler */
 void fIrqRtcHandler(void)
 {
     fRtcHandler();
 }
+#endif
 
 /** Call the I2C IRQ handler */
 void fIrqI2CHandler(void)

targets/TARGET_ONSEMI/TARGET_NCS36510/rtc.c

@@ -41,6 +41,9 @@
  * if the alarm occurs upon a Counter increment condition.
  *
  */
+
+#ifdef DEVICE_RTC
+
 #include "rtc.h"
 #include "mbed_assert.h"
 #include "lp_ticker_api.h"
@@ -300,3 +303,5 @@ boolean fIsRtcEnabled(void)
         return False;
     }
 }
+
+#endif